1. Field of the Invention
The present invention relates to thin film structures which include aluminum. In particular, it relates to metallurgies formed atop semiconductor substrates, such as silicon semiconductor metallization to form ohmic contacts, Schottky Barrier diode contacts and FET gate-type structures.
2. Description of the Prior Art
The requirements for a material or combination of materials to provide ohmic and Schottky barrier contacts to silicon semiconductor substrates are very stringent from both an electrical as well as a chemical standpoint.
Among the numerous metallurgical systems which are known to semiconductor designers for such contacts, the single most successful metal used in interconnections of silicon planar transistors and integrated circuits is aluminum or aluminum which is doped with a small amount of copper. However, as is well known in the art, aluminum has a tendency to interact with silicon, particularly during high temperature processing. In addition, aluminum alone forms neither a very high-nor a low barrier height Schottky barrier contact to silicon. With the usual aluminum-silicon barrier, a barrier height in the order from 0.68 to 0.72 electron volts is achievable.
This system can therefore not be used for forming a Schottky barrier diode (SBD) with a low barrier height, i.e., around 0.5 electron volts. Quite recently, circuit families such as DTL, I.sup.2 L and Schottky transistor logic have been implemented which require Schottky Barrier diodes with such low barrier height characteristics. Reference is made in particular to the DTL type circuit, described in the publication by A. W. Peltier entitled "Advances in Solid-State Log.c - A New Approach to Bipolar LSI: C.sup.3 L," 1975 IEEE International Solid-State Circuits Conference, Digest of Technical Papers, pages 168-169.
In addition, more sophisticated circuit designs require both a high-barrier-height Schottky barrier diode contact as well as a low-barrier-height Schottky barrier diode contact on the same semiconductor substrate. Most useful is a metal-lurgical system which would provide not only these types of contacts but also ohmic contacts, so that each type of contact could be fashioned concurrently during the fabrication process.
The above-referenced application of Dalal et al has solved these problems by providing a structure which comprises a bottom layer of tantalum in contact with the silicon substrate, an intermediate layer of chrome and a top layer of aluminum, the chrome being necessary to prevent the interaction of the aluminum with the tantalum or silicon substrate. Without the chrome diffusion barrier, this interaction causes the barrier height of the tantalum-silicon contact to increase, i.e., to approach that of an aluminum-silicon contact.
The Dalal et al contact structure has succeeded in providing a Schottky barrier diode of around 0.5 electron volts; it can also be used to provide a high barrier height contact when platinum silicide is formed in the silicon layer prior to the deposition of tantalum. In addition, the structure described in the above-referenced application is useful as an ohmic contact when formed on a highly-doped N type silicon surface.
The fabrication process invented by Dalal et al requires careful controls to prevent contamination of the tantalum; in particular, the formation of tantalum oxide is deleterious. Moreover, it is desirable to reduce the number of different metals required by Dalal et al to achieve a low-barrier-height Schottky barrier diode.
The above-referenced application of Howard et al utilizes an aluminum-transition metal contact structure to form very reliable Schottky barrier diodes. However, such a structure provides a low-barrier-height contact which is thermally stable of around 0.62 electron volts rather than around 0.5 ev.
Another metallurgical contact which has achieved widespread use in the industry to provide a Schottky barrier diode having a barrier height of around 0.5 electron volts is a tinanium-tungsten alloy. However, it is quite difficult to evaporate titanium-tungsten, thus limiting its application to sputtering processes. In addition, the barrier height of titanium-tungsten Schottky barrier diode tends to drift upward with increasing annealing temperatures.
Aluminum also reacts with other metals and metal silicides, sometimes beneficially and sometimes deleteriously. For example, in the above-referenced application of Howard et al. Aluminum-transition metal compounds are useful as Schottky Barrier contacts. On the other hand, aluminum will penetrate through other metals like tantalum or platinum silicide to an underlying layer such as monocrystalline silicon or polycrystalline silicon, which may cause erosion of the silicon surface.
Moreover, aluminum will interact with gold and silver, rare earth metals such as Gd, Ln, Y, Yb, Pr, etc., lead and tin solders and amorphous silicon.